Glossary and Acronyms: A

Absolute vorticity:

vorticity measured in an absolute co-ordinate system. In nearly all cases only the vertical component is implied. The absolute vorticity is the sum of the relative vorticity and the vorticity of the Earth (i.e. the Coriolis parameter). The absolute vorticity at 850 hPa is very useful to follow tropical system centres.

Air temperature:

the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation.

Asynoptic observation:

a supplementary meteorological observation taken between regular synoptic hours.

Atmospheric absorption:

transformation of radiant energy into thermal, mechanical, electrical, etc ... , energy, through its absorption by atmospheric constituents.

Adiabatic process:

thermodynamic transformation which occurs without the exchange of heat between a system and its environment. In an adiabatic process any change in internal energy (for a system of fixed mass) is solely a consequence of working. For an ideal gas and for most atmospheric systems, compression results in warming, expansion results in cooling.
Under the tropics, the adiabatic hypothesis is often not kept because the exchanges of heat within the atmosphere are numerous (for example, with the convective process where the release of latent heat due to condensation warms the atmosphere, or with the radiative transfer, when the temperature decreases in the atmosphere).

ADT:

Advanced Dvorak Technique. This technique utilizes longwave-infrared, temperature measurements from geostationary satellites to estimate tropical cyclone (TC) intensity. The ADT is based upon the operational Dvorak Technique developed by Vernon Dvorak of NOAA. The ADT currently utilizes an objective storm center determination scheme and cloud pattern determination logic to remove the subjectivity aspect from the intensity estimation process. It also can be applied to all phases of the TC lifecycle; something that previous objective schemes (ODT Objective Dvorak Technique and AODT Advanced Objective Dvorak Technique) could not do.
For further information, see: http://cimss.ssec.wisc.edu/tropic2.

ADEOS:

ADvanced Earth Observing Satellite (of NASDA, Japan).
A Japanese remote sensing satellite designed to collect worldwide environmental data from a sophisticated suite of sensors.
The core sensors on ADEOS are an advanced visible and near-infrared radiometer and an ocean color and temperature sensor. Additional instruments on ADEOS include the NASA scatterometer, a total ozone mapping spectrometer, an instrument to measure the polarization and directionality of the earth's reflectance, an interferometric monitor for greenhouse gases, an improved limb atmospheric spectrometer, and a retroreflector in space. ADEOS was launched on 17 August 1996, into a sun-synchronous orbit. The satellite failed on 29 June 1997, but will be replaced by other satellites in the ADEOS series.

Advection:

transfer of air mass properties by the velocity field of the atmosphere.

Aerosol :

solid particles or liquid droplets, held in suspension in the atmosphere.

AFP :

Agence France Presse. (French Press Agency).

AFWA :

Air Force Weather Agency.

Air mass :

An extensive body of the atmosphere whose physical properties, particularly temperature and humidity, exhibit only small and continuous differences in the horizontal. It may extend over an area of several million square kilometres and over a depth of several kilometres. Air masses are classified in terms of : latitude of origin (tropical, polar, arctic or antarctic), the nature of the underlying surface (continental or maritime), hydrostatic instability (stable or unstable), the difference between the temperature of an air mass and that of the underlying surface or of a neighbouring air mass (warmer or colder).

Aladin:

limited area model (LAM) coupled to the global model ARPEGE-Tropiques until March 2008 and then to the french global model ARPEGE since then. This LAM is only used over the SouthWest Indian Ocean, from the Equator to 32S and between 31.5E and 88.5E, and is operational since October 2006. The resolution is currently of 10 km, with 60 vertical levels, but at the first quarter of 2010, the resolution will be of 8 km, with 70 vertical levels. Based on observations of the wind between the surface and 500 hPa, analytical models can compute the corresponding three-dimensional cyclone structure, which bogusses the tropical cyclone.
There are two runs a day, at 00Z and 12Z, with ranges up to 84h.

Amount of precipitation:

depth to which precipitation in liquid form would cover a horizontal projection of the earth's surface, in the absence of infiltartion, runoff or evaporation. It is typically expressed in millimeters, equivalent to liters per square meter.

AMSU:

Advanced Microwave Sounding Unit. The AMSU instrument detects earth/atmosphere emitted radiation in the microwave portion of the electromagnetic spectrum. Microwaves, in comparison to more familiar infrared or visible radiation, are less energetic and have longer wavelengths (distance between successive wave crests/troughs) on the order of centimeters (10^-2 meters) vs. micrometers (10^-6 meters). Based on energy considerations and AMSU instrument performance, this dictates that the sensor be placed in a lower earth-orbit (~810km above the earth's surface vs. ~36,000km for geostationary satellites) to improve instrument signal-to-noise.

The AMSU-A (temperature sounder), a 15 channel passive radiometer, detects energy emitted by atmospheric molecular oxygen (a major atmospheric constituent) and is largely unaffected by the presence of clouds--from the emission source, through the atmosphere, to the sensor which resides on the NOAA, NASA/Aqua and METOP polar orbiting satellites.
The AMSU-B is a five-channel microwave instrument, flown to measure profiles of atmospheric humidity.

For further details, see http://amsu.ssec.wisc.edu or http://amsu.cira.colostate.edu or http://aqua.nasa.gov/about

AMSR-E:

Advanced Microwave Scanning Radiometer for EOS (Earth Observing System). is a conically scanning passive microwave radiometer sensing microwave radiation (brightness temperatures) at 12 channels and 6 frequencies ranging from 6.9 to 89.0 GHz. This instrument is aboard the satellite AQUA (NASA).

Analysis:

in synoptic meteorology, a detailed study of the state of the atmosphere over a particular region, based on the actual observations. The Dvorak analysis gives the intensity of a tropical system, thanks to the Dvorak technique.

Anemometer:

instrument which measures wind speed or wind speed and direction. The anemometer level is the height above ground at which an anemometer is exposed on the anemometer mast (or tower).

Anticyclone:

region of the atmosphere where the pressures are high relative to those in the surrounding region at the same level. If nothing else is specified, it means it is a region at the surface level.
At upper levels the regions of higher pressures are regions of high geopotential or upper level anticyclones (or ridges).
An anticyclonic circulation is an atmospheric circulation sytematically associated with an anticylone, which is clockwise in the northern Hemisphere, and anti-clockwise in the Southern Hemisphere.
In the tropics, an upper level (at about 200 hPa) anticyclone (or ridge), located above an area of cyclogenesis or a tropical system favours the upper level divergence and the outflows and so favours the intensification of the tropical system.

AODT:

Advanced Objective Dvorak Technique. See ADT.

APT :

Automatic Picture Transmission. An analog image transmission system developed for use on weather satellites. It was introduced in the 1960s and over four decades has provided image data to relatively low-cost user stations at locations in most countries of the world. A user station anywhere in the world can receive local data at least twice a day from each satellite as it passes nearly overhead.

(RSMC La Réunion) Area of Responsibility (AoR):

South-West Indian Ocean, extending between the Equator and the 40°S, and from the african coastline to the 90°E.
East of 90E and south of 10S, the area is monitored by the TCWC (Tropical Cyclone Warning Center) Perth (Australia), whereas north of 10S, it is monitored by the TCWC Jakarta (Indonesia).
Until the cyclone season 1984/1985, the systems coming from the Australian AoR were renamed when crossing the 80E (at this time, La Réunion Centre did not monitor the area east of 80E).
First extending from the 05S to the 30S, the area has then been extended from the Equator to the 40S in 2003 at the TCC in Maputo (Mozambique) for the RSMC to handle "non standard" systems.

Arpège:

(or Arpège-IFS). ARPEGE-IFS is a common development between Météo-France and ECMWF. ARPEGE (Action de Recherche Petite Echelle Grande Echelle) is the French name while IFS (Integrated Forecast System) is the name used by ECMWF. It is a global stretched model with no tropical cyclone's initialization. There are 4 runs a day : 00, 06, 12, 18 UTC. 00 UTC run is up to range 102h, 06 UTC up to 72h, 12 UTC up to 84h, and 18 UTC up to 60h. The time step is 900 seconds. The assimilation runs with a 6-hour cycle, and the analysis method is a four dimensional variational analysis for upper air fields. The resolution varies from 15 km over France to 55 km over Reunion Island and 90 km over New-Zealand.

ATCF :

Advanced Tropical Cyclone Forecast System.

Atmospheric circulation:

atmospheric motions above the Earth's surface. The atmosphere is the gaseous envelope which surrounds the Earth.

Atmospheric disturbance:

According to the context:
1. Any interruption of a state of equilibrium of the atmosphere.
2. Minor depression or meteorological conditions in an area showing signs of a developing cyclonic circulation.

Atmospheric pressure:

Pressure (force per unit) exerted by the atmosphere on any surface by virtue of its weight; it is equivalent to the weight of a vertical column of air extending above a surface of unit area to the outer limit of the atmosphere.
101 325 pascals = 1013,25 hPa = 1013,25 millibar. 1013,25 hPa of atmospheric pressure is equivalent to 10 000 kg of pressure on a surface of unit area.
In the standard atmosphere adopted by the International Civil Aviation Organization, the standard sea level pressure is 1013.25 hPa (or 760 mmHg) at 15°C.
In the standard atmosphere, the variation of the atmospheric pressure is about -1hPa/10m from the sea level up to 2.000 m, then -1hPa/15m from 2.000m up to 10.000m, and the variation of the standard temperature is about - 7°C/km.

Atmospheric tide :

Periodical oscillations of the atmosphere, caused by the gravitational action of the Moon and the Sun. Is more marked in the tropical areas, and very weak and often hidden by largest variations of pressure in the mid-latitudes regions. Its exact amplitude depends on the place and the date. At la Reunion Island, its amplitude is around 2 hPa, with two minimums near 04 hours and 16 hours local time, and two maximums near 10 hours and 22 hours local time. It is preferable to take account of this atmospheric tide for the estimation of a tropical low intensity, particularly at the onset of its life.

ASCAT:

Advanced SCATterometer. The Advanced Scatterometer (ASCAT) is an active (radar type) instrument designed to measure wind speeds and direction over the open ocean. It is being developed by ESA for flight on the Metop satellites, and is derived from the successful scatterometer flown on ESA's ERS-1 and ERS-2 satellites.
ASCAT has double 500 km-wide swathes, located 350 km either side of the Metop spacecraft ground track and is able to provide quasi-global data coverage within 24 hours. Three measurements (a so-called sigma-naught triplet) of the backscatter coefficient will be obtained at each measurement node, which are on a regular grid spaced at 25 km (low resolution) and 12.5 km (high resolution). The spacing of the resulting wind vectors after processing will be 50 km or better. (source WMO).

AVHRR:

Advanced Very High Resolution Radiometer. Multi-spectral imaging instrument aboard american NOAA (onwards NOAA-K) and EUMETSAT's Metop satellites. From each satellite it provides global cloud imagery twice a day, as well as providing frequent images of land and sea surfaces. (source WMO).

AVN:

AViation Model. AVN is the aviation run for the National Centers for Environmental Prediction (NCEP) Medium Range Forecast (MRF) global model. The AVN runs 4 times per day, starting with analyses valid at 00, 06, 12 and 18 UTC.